Ultrasonic atomization techniques are currently available for forming drops of liquid that have number median drop sizes (dN,0.5) of slightly below 20 microns (i.e., approximately 17 or 18 microns). According to these techniques, a solid surface of a metallic nozzle is vibrated at an ultrasonic frequency. Then, a liquid is introduced onto the surface of the nozzle and forms a liquid film thereon.
Since the solid surface vibrates in a direction that is perpendicular to the surface the liquid film, the liquid film absorbs vibrational energy from the solid surface. As a result, standing waves (known as “capillary waves”) form in the liquid film. These capillary waves form a rectangular grid of wave crests and troughs and, at relatively low amplitudes of a given vibrational frequency, the crests and troughs of the standing waves are uniformly distributed and stable. However, as the amplitude of the given vibrational frequency is increased, the distance between the crests and troughs of the capillary waves increases (i.e., the waves grow larger) until, at a critical amplitude, the waves become unstable and collapse.
As unstable waves collapse, drops of liquid are ejected from the crests of the waves. These drops are ejected at a low velocity in a direction that is normal to the vibrating, solid surface. The formation and ejection of these drops is referred to as “ultrasonic atomization.”
The range of amplitudes over which atomization occurs at a given frequency is limited. As discussed above, when the amplitude of the vibration is below a critical level, the capillary waves are stable and no appreciable amount of liquid is ejected from the crests of the waves. On the other hand, when the amplitude is too far above the critical level, cavitation occurs, wherein relatively large amounts of liquid are ejected at high velocities from the vibrating surface. Since cavitation is undesirable when relatively small drops of liquid are sought, when implementing currently-available ultrasonic atomization techniques, the amplitude of vibration is maintained within a relatively narrow range.
The peak-to-peak distance between any two adjacent crests in the above-discussed stable, capillary waves depends upon the frequency at which the solid surface vibrates. For example, adjacent crests form in closer proximity to each other at high frequencies than they do at lower frequencies. As such, when capillary waves become unstable and collapse, waves having adjacent crests that are closer together eject smaller drops of liquid than do waves having adjacent crests that are further apart from each other. Therefore, when the formation of relatively small drops of liquid is sought, it is often desirable to operate an ultrasonic atomization device at a relatively high frequency.
One currently-available ultrasonic atomization device that may be used to implement the above discussed techniques includes a nozzle that itself includes three principle active sections: an atomizing section (i.e., a front horn), a rear section (i.e., a rear horn) and an intermediate section. The front horn includes a solid, metallic vibrating surface where atomization takes places. The rear horn is configured to be connected to a source of liquid to allow the liquid to enter the nozzle. The intermediate section, which is positioned between the front horn and the rear horn, includes two piezoelectric transducers. When in operation, these transducers cause the atomizing surface on the front horn to vibrate at an ultrasonic frequency. More specifically, the transducers convert high-frequency electrical energy from an external power source into high-frequency mechanical motion that is transferred to the atomizing surface in order to cause the vibration thereof.
The transducers in currently-available ultrasonic atomization devices are disk-shaped and made from zirconate-titanate ceramics. Also, silver-plated or nickel-plated copper electrodes are used to introduce high-frequency electrical energy into the currently-available nozzle.
The front and rear horn of the currently-available nozzle are each fabricated from a Ti-6Al-4V titanium alloy. However, like all metal-based nozzles, this alloy has a plurality of shortcomings when it comes to forming small drops of liquid via ultrasonic atomization techniques. For example, the number median drop size (dN,0.5) of the drops formed has a lower limit of approximately 17 or 18 microns. Also, the maximum flow rate of the liquid from which such small drops may be formed has an upper limit of approximately 10 gallons per hour (i.e., 600 ml per minute).
At least in view of the above, it would be desirable to provide nozzles and methods capable of forming drops of liquid having a number median drop size below 17 or 18 microns. It would also be desirable to provide nozzles and methods capable of forming such drops while maintaining flow rates of above 10 gallons per hour.